This study is carried out under the assumption that the effective depth of root zone is not restricted to the conventional predefined root depth. Following the reduction of soil moisture within the root zone water uptake potential gradient between the soil in the rootzone and that underneath enables the root to use the available moisture from the soil deeper than the root zone. Based on this assumption, a lysimeter study was conducted in a complete randomized block design of three treatments (A, B, C) and three replications on soybean crop plant. The difference among irrigation treatments was related to irrigation scheduling (amount and interval of irrigation). Treatment A (control) was designed for irrigations when 55% of available moisture was depleted (MAD=55%) and the depth of irrigation was enough only to compensate for moisture deficit. Treatment B (delayed) was set at MAD=65% and net irrigation the same as that in treatment A. Treatment C (excess) was set also at MAD=65% but accompanied by 30% of extra water. The yield under each treatment was recorded and statistically analyzed. Results of soil moisture monitoring throughout the growing season showed that treatment B was faced with water stress due to excess water depletion, delayed irrigation and absence of adequate moisture storage in soil depth. In treatment B, some such yield traits such as number of pods, grain weight in pods, and grain yield were less than those obtained in treatment A. However in treatment C, in spite of the fact that MAD was set similar to treatment B and irrigation interval extended, no signs of water stress were observed in the plants. In addition the yield was not significantly different that that under treatment A. In treatment C, extra amount of irrigation, often assumed to be lost, was preserved beneath the root zone. When water in the upper soil layers was depleted, the reserved water was accessible to the plant which was able to take up the excess water to protect itself from water stress. Such observations show that presence of soil moisture beyond the root zone could be critically beneficial under water stress conditions. Finally, based on the assumed irrigation efficiency of 70% the total amount of applied water would be equal to 404.7 mm. At treatment C, beneficial use was equal to 328.7 mm for which actual irrigation application efficiency would be equal to 81.2%. This comparison shows that from the assumed 30% water loss, an extra 11.2% was available for plant water use. This phenomenon indicates that taking into account of moisture preserved under root zone can somehow increase irrigation efficieancy.